For pulling conductors at overhead transmission and distribution lines, stringing blocks are installed at the line structures. A pilot line is stringed through the blocks, either manually or by a helicopter. Then, the pilot line is exchanged by a single or bundled conductor, running on the blocks. Finally, the conductor is sagged and then transferred from the blocks to clamps.
The most of existing blocks consist of one or more sheaves, installed on an axle via bearings, the axle is supported by a frame, suitable for to be hanged to a line structure or an insulator. The frame is heavily loaded, especially at the blocks for helicopter stringing or for bundled conductor installation, and, accordingly, heavy weighting. Some kinds of stringing blocks are disclosed in the U.S. Pat. No. 4,018,422 by Bozeman, U.S. Pat. No. 4,129,287 and U.S. Pat. No. 4,301,994 by Lindsey.
A significantly lighter stringing block is disclosed in the U.S. Pat. No. 4,201,369 by Betta, but it doesn't support helicopter stringing.
At suspension structures with line angle not far from 180° (direct line), the block weight is less problematic: hard work for linemen. In contrast, at angle structures, the block, when stringing the pilot line, turns to an inclined position, and its weight is supported by the pilot line or conductor. It causes two main problems:    a) The relatively light pilot line has a tendency to escape the groove of a sheave of the stringing block. It also happens with light conductors, requiring big blocks, like optical ground conductor and other communication cables. The phenomenon is mentioned in the U.S. Pat. No. 4,420,143 by Lindsey and in the catalogue of Sherman & Reilly, Inc.    b) Conductor friction at the sheave flange takes place, causing the conductor damage.
One way to solve the problem of escaping the pilot line from the groove is fixing the block to the structure in an oblique position, rather than hanging it. A number of embodiments of this solution can be found in the catalogue of Sherman & Reilly, Inc. If the block installation angle is correctly calculated for given conductor weight and tension and line angle, it really solves the jumping problem. But since the weight of the pilot line is different from that of the conductor, and the tension is variable through the pulling process, fixing the block position exacerbates the second problem, increasing the conductor friction on the sheave flange.
Another way is disclosed in the U.S. Pat. No. 5,221,074 by Saracini. A plurality of small rolls, forming together a conductor route, similar to circumference of a single sheave, is used instead of the full-size sheave. These blocks are compact and relatively light. The suppliers of such kind of stringing blocks argue, that the conductor bending on these blocks is the same as on regular ones. But a dominant opinion is that the conductor is strongly bended and unbended on each single roll, and damaged; as a result, many conductor suppliers explicitly forbid using these blocks with their conductors.
One more way to solve this problem is disclosed in the U.S. Pat. No. 5,941,507 by Page. Two rolls are placed at the both sides of the sheave and accept the side pressure of the cable instead of the groove side. One problem with this block is that sensible cables cannot run on rolls (Page points that this block is intended for heavy cables). Another one is that it doesn't support helicopter stringing.
It would clearly be beneficial to provide a stringing block, intended for hanging to electrical line structures, insulators etc., suitable for both manual and helicopter pilot line or conductor stringing, of such a light weight, that said two problems will be solved.